The treatment of organic wastes such as municipal sludge and industrial wastes has become of increasing importance in our society. One method for treating such organic waste is a wet oxidation reaction system using conventional above ground equipment or a subterranean or "down-hole" reactor.
The first known successful subterranean wet oxidation reaction apparatus was constructed and operated by the assignee of the present application according to the principles set forth in McGrew, U.S. Pat. No. 4,272,383, which is hereby incorporated by reference. This down-hole reaction apparatus has a vertical configuration which utilizes gravitational forces in thermodynamic relationships to provide a high pressure reaction environment at optimal mass transfer.
This down-hole reaction apparatus is particularly useful in breaking down organic matter present in the large volume municipal waste market through an aqueous phase combustion process which is generally referred to as "wet oxidation." As will be known to those skilled in the art, wet oxidation of combustible organic matter is an exothermic reaction which proceeds rapidly at temperatures above 200.degree. C. The reduction of the chemical oxygen demand (COD) of the waste is a primary goal of the municipal waste destruction process. By reducing the waste COD, eutrophication of the receiving waters is prevented or, at least, minimized. In addition, the wet oxidation process degrades potentially toxic hydrocarbons which could otherwise pollute receiving waters. Thus, wet oxidation is a proven method for the destruction of municipal organic wastes and industrial organic wastes.
Generally, a down-hole reactor apparatus comprises a vertically oriented, subsurface chamber defined by the casing of the subterranean shaft. The subterranean shaft normally extends about 500 to 3000 meters and preferably about 1200 meters into the earth. Suspended in the chamber and spaced apart from the casing is a tubular reaction vessel. The tubular reaction vessel has a closed end waste containment tube in which a waste pipe is centrally disposed. The containment tube and the waste pipe are arranged concentrically to form an external passage or annulus defined by the inner wall of the containment tube and the outer wall of the waste pipe. The bore of the waste pipe and the external passage are in full communication at the lower end of the reaction vessel. Also suspended in the chamber is a conduit which is substantially parallel to, but spaced apart from, the reaction vessel. Through this conduit, a heat transfer medium is preferably pumped into the chamber. Thus, an external heat exchanger is provided.
In order to rapidly oxidize the quantities of organic matter found in municipal or industrial wastes, it is necessary to supply an oxidant. Hence, during operation of a wet oxidation system municipal or industrial wastes which contain organic matter are pumped or injected into the down going reaction passage along with an oxidant. The oxidant can be air, oxygen-enriched air, or essentially pure gaseous oxygen. Generally, oxygen-enriched air or pure gaseous oxygen are preferred. Liquid oxygen can be supplied on site for conversion into the gaseous state for injection as needed. The flow rates of the diluted municipal or industrial wastes and the gaseous oxidant or oxidants through the reaction apparatus are regulated to provide a sufficient flow velocity such that intense mixing occurs. Such mixing enhances the mass transfer between the oxygen and the combustible components of the municipal or industrial waste. The gaseous oxygen may be injected into a diluted municipal or industrial waste through one or more gas supply lines which are suspended in the down going and/or upcoming reactant passage.
As the concentration of available oxygen and the temperature of the reactants increase, the rate of the wet oxidation reaction also increases. The exothermic oxidation reaction generates substantial heat which, in turn, further elevates the temperature of the reactants. Given the supply of oxidizable material is not limiting, and the temperature of the reactants reaches about 180.degree. to 210.degree. C., the reaction becomes autogenic. The hydrostatic head of the column of the diluted waste prevents the aqueous reaction mixture in the reaction zone from boiling. Typically, the column or diluted waste and the gaseous oxygen are mixed throughout the entire length of the reaction apparatus, which can exceed a kilometer in length. The temperature of the reaction mixture is allowed to increase to about 260.degree. to 370.degree. C. within the reaction zone (the lower part of the down-going reactant passage). The diluted municipal or industrial waste is thereby oxidized in the wet oxidation reaction. The reaction products or a solid containing effluent of this oxidation process include a low volume, sterile solid residue material, a liquid effluent, and off gases.
As will be apparent to one skilled in the art, one key to the commercial success of such vertical tube reaction systems is the energy efficiency possible as a result of employing the natural principals of gravity and thermodynamics to create the heat and pressure necessary to maintain the reaction. To insure the necessary efficiencies of the system, it is important that the walls of the tube remain substantially free of inorganic scale and that other accumulations or plugging within the reactor tubes are minimized. Scale build-up on the walls of the vertical tubes reduces the efficiency of the heat exchange process between the influent and the effluent waste streams through the walls of the tubes separating the two flows, thereby reducing both the energy recovery and control over the reaction process. Scale build-up on the walls of the tubes also increases wall friction and reduces the available cross-sectional area through which the fluid waste streams may flow, thereby increasing the load on the pump or pumps injecting and/or circulating the fluid waste and oxidant streams.
The scale found in a wet oxidation treatment apparatus consists, in a large part, of hard anhydrite scale which is comprised of calcium sulfate (CaSO.sub.4) Al-phosphates (apatite). The environment of a wet oxidation process will include pyrolysis, hydrolysis, and oxidation reactions. In such an environment, the retrograde solubility of calcium sulfate will result in the precipitation of calcium sulfate and the formation of anhydrite scale on the hot tube surfaces. Scale deposition will be especially heavy on the tube surfaces found in the hottest portion of the reactor system (i.e., the reaction zone). With calcium sulfate and other substances which exhibit such retrograde solubility, as the solution temperature increases, the solubility decreases. Thus, in the reaction zone, which is at a temperature of approximately 260.degree. to 370.degree. C. essential quantities of calcium sulfate present will precipitate out of the solution, thereby forming scale.
Periodic removal of such scale is necessary in order to minimize degradation of fluid flow and heat transfer characteristics of the reactor. Scale removal is usually accomplished by taking the reactor out of service and circulating a cleaning chemical or chemicals through the reactor. These cleaning chemicals or solutions act as solvents to dissolve and suspend as a slurry the scale to the point where it can be readily removed or detached from the metal surfaces and flushed from the reactor system. The descaling chemicals normally used are ones which will readily remove calcium and aluminum compounds and especially calcium sulfate. Such descaling chemicals include nitric acid, ethylene diamine tetraacetic acid, and Rochelle salts. Nitric acid is the most commonly used solvent for removing scale in a wet oxidation reactor. In such a descaling process, nitric acid tends to act as a solvent rather than as an acid/base reactant. Therefore, the nitric acid is not neutralized during the descaling procedure. The prevailing practice is to neutralize the nitric acid contained in a used or spent descaling solution prior to disposal by reacting it with a base such as an oxide, hydroxide, or carbonate of sodium, calcium, or ammonia. The resulting neutralized effluent may still contain significant amounts of nitrogen-containing compounds.
The disposal of such effluents creates considerable problems due to the nitrogen-containing materials therein. Normally such materials cannot be discharged to surface or ground water or to publicly owned treatment works (POTWs) because of nitrogen limits placed on such waters and discharges flowing to them. It is desired, therefore, to provide a method by which the nitrogen content of such resulting descaling solutions could be reduced. The present invention provides a method by which the nitrogen content of the resulting descaling effluent can be significantly reduced to more environmentally compatible levels and also shows that nitrogen present in the waste is reduced by the aqueous phase oxidation.